The present invention relates to remote control of a voltage applied to an electric load, and specifically to dimming of a halogen lamp by means of an on/off switch in the lamp circuit.
It is well known in the art that the light output from an incandescent lamp can be varied by changing the voltage applied to the lamp. Known lamp dimmers generally use a remote potentiometer, which controls the conduction angle of a triac in the AC circuit of the lamp. A triac is a solid state switch similar to a thyristor.
U.S. Pat. No. 4,837,455 to Sleator describes how separate loads controlled by electromagnetic relays, solid state switching devices, or electrothermal relays can be selected remotely by flipping an on/off switch common to all the loads rapidly off and on again. An incandescent lamp is described as a possible load, but Sleator contemplates only on/off switching of the lamp.
Today, there are devices on the market that use rapid operation of an on/off switch for remote control purposes of lamps. One such device, which is designed to be inserted between an ordinary light bulb and a light bulb socket, makes it possible to switch the lamp from burning steadily to flashing by flipping an on/off switch in the lamp circuit off and then immediately on again. Another device of similar construction makes it possible to dim a lamp in one or more steps by rapid operation of an on/off switch.
All the known devices for dimming of a lamp use triacs to control the current flow in the lamp circuit. Triac based dimmers produce high frequency noise, which must be filtered out to avoid radio and TV interference, and they also generate undesirable DC components in the AC circuit.
Halogen lamps are low voltage devices, so they require a transformer between the house power and the light bulb. Toroidal transformers are the preferred type of transformers for halogen lamps, because they are small and light weight so that they are easy to locate in the lamp armature. A toroidal transformer has a core of toroidal configuration made by a tape-wound strip of electrical steel. Primary and secondary windings are threaded through the central opening in the toroidal core and distributed evenly along the circumference of the core. Toroidal transformers have air gap free cores, so they are sensitive to DC components in the transformer circuit. A triac based dimmer, which generates DC components, is accordingly not a viable option for control of halogen lamps with toroidal transformers.